The present invention relates to an electrostatic printing apparatus (an electrophotographic apparatus) utilizing an electrophotographic method, such as a copier, a printer and the like; and, more particularly, the invention relates to control of an erasing unit for removing residual charge on a photosensitive body using an erasing light.
An electrostatic printing apparatus utilizing an electrophotographic method has a construction in which various kinds of electrophotographic processing units, such as a charging unit, a light writing unit (an exposing device), a developing unit, a transferring unit, a separating unit, a cleaning unit, a discharging unit and so on, are serially arranged around a photosensitive body, and the charged photosensitive body is exposed based on image information to form an electrostatic latent image on the surface of the photosensitive body, and the latent image is converted to a visible image using toner.
A line printer provided as an electrostatic printing apparatus is required to have a printing speed which is higher with an increase of the information volume to be processed. In high speed printing, abrasion of the photosensitive body becomes large due to friction with the paper and the developer. In order to avoid such a large abrasion, an As.sub.2 Se.sub.3 group photosensitive body (Vickers hardness: Hv.apprxeq.150) has been widely used.
The As.sub.2 Se.sub.3 photosensitive body has a less stable residual electric potential compared to a Selenium Tellurium alloy photosensitive body. Therefore, when the same pattern is repetitively printed, the residual electric potential at that portion is increased. Then, when a pattern covering over both the portion having an increased electric potential and a portion unit having an increased electric potential is printed, a light portion and a dark portion appear in the image.
For example, when a whole black image is printed after thin lines are repetitively printed, there appears a phenomenon that the whole black image is bleached out in the portions corresponding to the thin lines which were repetitively printed just before. That is, a so-called residual image phenomenon takes place (refer to white bleached out portion in FIG. 2). The residual image phenomenon is conspicuously observed when an image having a low density is printed.
As a solution to the problem of the residual image phenomenon, it has been proposed to add iodine to an As.sub.2 Se.sub.3 photosensitive body or to thin the film thickness of the photosensitive body. However, in a high speed printing machine, even if the addition of iodine or the thinning of the film thickness of the photosensitive body is performed, light and dark portions are produced in a image due to an increase of the residual electric potential at the time of low density printing. The portions exhibiting a residual image phenomenon can be made less conspicuous by increasing the light intensity of the erasing light, but this approach is not preferable because the lifetime of the photosensitive body is shortened when a large amount of light is always irradiated thereon.